Magnetic resonance imaging (MRI) is a medical imaging modality that can create images of the inside of a human body without using x-rays or other ionizing radiation. MRI uses a powerful magnet to create a strong, uniform, static magnetic field (i.e., the “main magnetic field”). When a human body, or part of a human body, is placed in the main magnetic field, the nuclear spins that are associated with the hydrogen nuclei in tissue water become polarized. This means that the magnetic moments that are associated with these spins become preferentially aligned along the direction of the main magnetic field, resulting in a small net tissue magnetization along that axis (the “z axis,” by convention). An MRI system also comprises components called gradient coils that produce smaller amplitude, spatially varying magnetic fields when a current is applied to them. Typically, gradient coils are designed to produce a magnetic field component that is aligned along the z axis and that varies linearly in amplitude with position along one of the x, y or z axes. The effect of a gradient coil is to create a small ramp on the magnetic field strength and concomitantly on the resonant frequency of the nuclear spins, along a single axis. Three gradient coils with orthogonal axes are used to “spatially encode” the MR signal by creating a signature resonance frequency at each location in the body. Radio frequency (RF) coils are used to create pulses of RF energy at or near the resonance frequency of the hydrogen nuclei. The RF coils are used to add energy to the nuclear spin system in a controlled fashion. As the nuclear spins then relax back to their rest energy state, they give up energy in the form of an RF signal. The RF signal is detected by an RF receiver coil or coils and is transformed into an image using a computer and known reconstruction algorithms.
The amplitude of the signals detected by the RF receiver coil(s) are typically small. An RF receiver coil may be connected to a preamplifier that is used to amplify the signals detected by the RF receiver coil prior to further signal processing. In an MRI system with a plurality of RF receiver coils, a preamplifier may be provided for each RF receiver coil. Preamplifiers reduce loop to loop coupling and improve the system noise figure. A preamplifier, however, can radiate from its output interface and create an oscillator. A low loss, highly shielded output cable and connector may be used to minimize coupling and loss, however, such cables and connectors may not be size or cost efficient, in particular, in a system with a plurality of RF receiver coils. Therefore, there is a need for a system and apparatus to reduce or eliminate oscillation created by the preamplifiers.